This invention relates generally to digitally-controlled oscillators, and in particular to a digitally-controlled oscillator having progressively-sized inverter elements.
Digitally-controlled oscillators (DCOs) are oscillators having an output frequency that is adjustable by adjusting digital input control signals to the DCO. Conventional DCOs include a base oscillator that generates an output signal at a base frequency and frequency-adjusting stages connected to the base oscillator to adjust the frequency output from the base oscillator. Conventional DCOs suffer from various drawbacks including variations in output frequency steps with the activation of each subsequent frequency-adjusting stage.
FIG. 1 illustrates the differential non-linearity of a conventional DCO having twelve frequency-adjusting stages, where “differential” refers to the frequency difference when activating adjacent stages. When none of the stages are activated, the DCO outputs a base frequency. When one stage is activated, the DCO output frequency increases from the base frequency by F1. Activating stages 2 and 3 increases the DCO output frequency by F2 and F3 respectively. However, when a next stage is activated, the frequency increment drops to a value F4. As illustrated in FIG. 1, although the general trend is for the frequency of the DCO to increase as more stages are activated, the increment is not linear. In some conventional DCOs, the frequency increment or frequency step may oscillate by 10 percent or more of a nominal frequency step indicated by the least-significant-bit of a control byte, word, string or register.